Aerodynamic roughness of rippled beds under active saltation at Earth-to-Mars atmospheric pressures

TL;DR

This study measures the aerodynamic roughness length over rippled sand beds with active saltation at pressures between Earth and Mars, revealing it may be dominated by form drag and significantly larger than previously assumed.

Contribution

It provides the first robust measurements of $z_0$ over rippled beds under active saltation at intermediate atmospheric pressures, informing Martian wind and sediment transport models.

Findings

$z_0$ can reach up to 1 cm over rippled beds.

Form drag may dominate $z_0$ under active saltation.

$z_0$ is two orders of magnitude larger than flat bed assumptions.

Abstract

As winds blow over sand, grains are mobilized and reorganized into bedforms such as ripples and dunes. In turn, sand transport and bedforms affect the winds themselves. These complex interactions between winds and sediment render modeling of windswept landscapes challenging. A critical parameter in such models is the aerodynamic roughness length, $z_0$, defined as the height above the bed at which wind velocity predicted from the log law drops to zero. In aeolian environments, $z_0$ can variably be controlled by the laminar viscous sublayer, grain roughness, form drag from bedforms, or the saltation layer. Estimates of $z_0$ are used on Mars, notably, to predict wind speeds, sand fluxes, and global circulation patterns; yet, no robust measurements of $z_0$ have been performed over rippled sand on Mars to date. Here, we measure $z_0$ over equilibrated rippled sand beds with active saltation under atmospheric pressures intermediate between those of Earth and Mars. Extrapolated to Mars, our results suggest that $z_0$ over rippled beds and under active saltation may be dominated by form drag across a plausible range of wind velocities, reaching values up to 1 cm -- two orders of magnitude larger than typically assumed for flat beds under similar sediment transport conditions.